A Raman crystal is a nonlinear optical crystal that is capable of supporting a nonlinear optical interaction known as stimulated Raman scattering (“Raman interaction”). Lasers that have optical gain provided by a Raman interaction within a Raman crystal (“Raman lasers”) show promise.
Raman lasers may be used to generate light beams at a wavelength between approximately 1.4 μm to 1.8 μm—the eye-safe spectral region—and also outside of this wavelength range. Light beams at these wavelengths have many applications including laser range finding, scanning LIDAR, and remote sensing, provided that the light beams have sufficient power and spatial beam quality. A beam with high spatial beam quality may be made to be more intense than a beam with lesser spatial beam quality. Beams that have high spatial beam quality may deliver more power to the target and diverge less. Beams with high beam quality may be more tightly focussed and are also of benefit in many applications that require precise localization of the output beam.
Spatial beam quality of a beam may be characterised by at least one of the beam's M2 and the beam's intrinsic beam brightness Raman lasers having average output powers of up to 5 W and near diffraction limited beam quality (e.g. a beam with an M2 beam quality factor less than approximately 1.2) have been achieved, however, the intrinsic beam brightness of the output light beam is around one half or less of that of the input light. Spatial beam quality may be defined by the ISO Standard 11146, “Lasers and laser-related equipment—Test methods for laser beam widths, divergence angles and beam propagation ratios” (2005)
Consequently, till now it has been generally accepted that an input light that has an excellent beam quality should be selected for an output light beam to retain high brightness.
Sources of input light having a high intrinsic brightness may be relatively large, expensive, and generally inconvenient, and their use may be an impediment to the wider adoption of the technology. Sources of input beams having lower intrinsic beam brightness, however, may be more practical and cost effective. To date, however, the use of these sources has not resulted in a Raman laser that can emit an output light beam having high intrinsic beam brightness.